The proposed research program involves an indepth study of cylic AMP and calcium dependent chloride secretion by isolated rabbit colon, canine tracheal epithelium and Na Cl absorption by flounder intestine. These studies include a) further evaluation of a cellular model for active chloride secretion which appears to be applicable to a variety of secretory epithelia b) determination of the intestinal cell type responsible for secretion c) determination of the electrical potential profile of secretory cells and the relative resistances and permeabilities of their limiting cell membranes d) determination of intracellular ionic activities of secretory cells and e) evaluation of the role of cell calcium in mediating the effects of various secretory stimuli. The overall aim of this project is to characterize the mechanisms responsible for chloride secretion by epithelia and to evaluate the role of sodium chloride cotransport in this process. Sodium chloride cotransport is also involved in electrolyte absorption by flounder intestine and the properties of this transport process are being evaluated with both flux studies and electrophysiologic techniques. In both secretory epithelia and in flounder intestine a Na/K/C1 cotransport process may be involved.